The invention relates to an agent and process for the determination of calcium in liquids, especially in body fluids.
The determination of calcium in liquids using orthocresolphthalein complexone is a customary method in many industrial and clinical laboratories. The formation of a red complex with calcium was first described in 1955 (Analyst 80, 713 (1955)). Thereafter the method has been continually modified, so that in the meanwhile it can also be carried out on systems using continuous measurement, centrifugal analyzers and instruments carrying out discrete measurements.
The reference method for the determination of calcium is atomic absorption, but this is very time consuming. Today, flame photometry is generally used for determination in serum in clinical laboratories, but this requires a suitable flame photometer because of interference by sodium ions. As a rule, photometric determinations are only suitable for determining calcium in serum; they are susceptible to trouble and do not exhibit a good agreement with atomic absorption or with determination by flame photometry. A further disadvantage of the known photometric methods of determination can be seen in the fact that potassium cyanide is used (German Patent Specification No. 2,335,350), from which on acidification, hydrogen cyanide which is lethally toxic can be formed.
A reagent for the spectrophotometric determination of calcium in the presence of a zwitterionic buffer is disclosed in U.S. Pat. No. 3,822,116. The buffer substances mentioned therein, and also glycine which is preferentially used, do not have the buffer capacity required in the pH range necessary for the color reaction, so that the sensitivity of this method is considerably reduced.